1. Field of the Invention
The present invention relates generally to the field of geophysical surveying. More particularly, it concerns seismic methods and geophysical survey systems for petroleum and gas exploration that rely on an explosive seismic energy source.
2. Description of Related Art
Seismic geophysical surveys are used in petroleum and gas exploration to map the following: stratigraphy of subterranean formations, lateral continuity of geologic layers, locations of buried paleochannels, positions of faults in sedimentary layers, and basement topography. Such maps are deduced through analysis of the nature of reflections and refractions of generated seismic waves from interfaces between layers within the subterranean formation.
A seismic energy source is used to generate seismic waves that travel through the earth and are then reflected by various subterranean formations to the surface of the earth. As the seismic waves reach the surface, they are detected by an array of seismic detection devices, known as geophones, which transduce waves that are detected into representative electrical signals. The electrical signals generated by such an array are collected and analyzed to permit deduction of the nature of the subterranean formations at a given site.
Seismic energy sources that have been used in geophysical survey methods for petroleum and gas exploration include impact sources, gun sources, vibratory sources and explosives. The nature of output seismic energy depends on the type of seismic energy source that was used to generate it.
Fundamentally, an impact source is a weight striking the surface of the earth directly or impacting a plate placed on the earth""s surface, yielding seismic energy. A weight-drop is an example of the former type of impact source. While impact sources tend to be relatively inexpensive and simple to operate and maintain, their principal disadvantage is that they are inefficient at producing seismic energy useful for geophysical survey of deeper layers. Impact sources yield a relatively high proportion of low frequency, surface waves and output less seismic energy than other seismic energy sources.
Gun sources, like impact sources, transfer kinetic energy into seismic energy. They rely on the sudden, powerful release of a charge of pressurized gas, usually compressed air from an air gun, to generate seismic waves. Gun sources have an advantage over impact sources in that they produce more seismic energy than is possible with simple impact sources. The seismic energy generated by gun sources also tends to be of higher-frequency than that imparted by impact sources, and this helps to minimize surface wave generation and improve resolution. However, gun source equipment tends to be more bulky and expensive than simple impact sources.
Vibratory sources are also used as seismic energy sources in geophysical survey methods. Two categories of vibratory sources include those that generate seismic waves originating at the surface and those that generate seismic waves that emanate from downhole. One mechanical-hydraulic vibratory source, the VIBROSEIS truck, is specially designed to place all of its weight onto a large platform which vibrates. This vibration, in turn, produces seismic waves in the subterranean formation. VIBROSEIS trucks have been used extensively in geophysical survey methods, not just for the petroleum and gas exploration, but also for studying the evolution and development of specific geological structures (e.g., the Rocky Mountains) and fault lines. Vibratory sources tend to produce highly repeatable seismic energy. The nature of the energy delivered into the ground by vibratory sources, its amount, duration, and time of delivery, can be tightly controlled and therefore the seismic energy generated tends to be very reproducible, which is a benefit. However vibratory sources are often not suited to certain types of terrain. For example if the ground is very soft, it can be difficult to use VIBROSEIS trucks as a seismic energy source.
Another type of seismic energy source used in geophysical survey relies on explosives. Explosive seismic energy sources used in petroleum and gas exploration on land rely on the explosion of material placed within a subterranean formation to generate seismic waves. Typically, a hole is drilled in the ground, the explosive is placed in the hole, and backfill is piled on top of the explosive, prior to initiating the explosion. Compared on a pound for pound basis to gun sources and impact sources, explosive sources impart the highest amount of seismic energy into the ground. Explosive seismic energy sources currently being used in geophysical survey methods generally produce waves of very high frequency. They are often used when the ground conditions are such as to prevent the effective use of impact or gun sources (e.g., when the ground is extremely soft). In transition zone applications, since the sources are positioned in shallow holes, there is an environmental concern about the explosive blowing up the hole and creating an environmental scar. This can limit the amount of explosive that can be used and therefore the quality of the seismic data.
At present, the demand for seismic exploration methods that generate sharper energy pulses, which can result in higher resolution images, has led to sacrificing the generation of low frequency seismic waves. This loss of low frequency waves (e.g.,  greater than 3 seconds) compromises the ability to image deeper targets. While VIBROSEIS has been used successfully in mapping deeper targets, it has been difficult to achieve the same quality of results using explosive seismic sources. This presents a significant problem when there is a need for mapping deeper subterranean formations but the ground conditions are not suited to VIBROSEIS. In the past, the response has been to drill deeper boreholes and use more explosive material to achieve the desired results at such difficult mapping sites. Both drilling deeper and using more explosive material substantially increases the cost of subterranean mapping of a particular site.
There is a need for improved seismic methods and geophysical survey systems that efficiently generate low frequency seismic waves when needed. Furthermore, it would be advantageous to be able to use shallower boreholes and less explosive material to achieve the necessary level of data resolution for geophysical surveys.
This invention provides improved seismic methods and geophysical survey systems that are well suited for petroleum and gas exploration, but could be used for other purposes as well.
One embodiment of the invention is an apparatus for generating seismic waves comprising a housing, a plurality of explosive elements enclosed within the housing, and at least one barrier element separating the explosive elements. The barrier element can comprise an inert material, a non-explosive material or an explosive material having reactive properties different than the explosive elements. The apparatus can further comprise at least one detonation device where the detonation devices are located within the explosive elements and are connected to a detonation controller device capable of imposing controllable time delays between individual detonations. The explosive elements can comprise shaped charges.
Another embodiment of the invention is a seismic method that comprises the steps of generating seismic waves by detonating an explosive device in a subterranean formation, and detecting the seismic waves and/or reflections thereof with seismic detectors. The explosive device used in this method comprises a plurality of explosive elements such that the individual elements explode at differing times. The explosive device can be constructed in a manner so that it can be placed in a borehole within the subterranean formation, and covered with backfill before being exploded.
In yet another embodiment of the invention, the explosive elements are separated by a barrier material that can delay the detonation of adjacent explosive elements, such as a non-explosive flammable material. The explosive device can include a housing that encases the explosive elements and the barrier material and can also include one or more detonation devices with which to initiate the explosions. Each detonation device is typically connected to a detonation control device that activates the individual detonation devices.
In still another embodiment of the invention, the explosive elements are separated by a barrier material that will prevent the detonation of adjacent explosive elements, such as with an inert material. The explosive elements are then individually equipped with detonation devices that can be activated individually by a detonation control device at the surface. The detonation control device can have the ability to activate the detonation devices with pre-selected or controllable time delays between the individual detonations. There can be a plurality of detonation devices that are each connected to an explosive element where the detonation control device detonates the explosive elements at different times. The time delay between detonations can be less than 100 milliseconds and typically is between 0.1 and 100 milliseconds. The explosive elements can optionally comprise shaped charges that can enhance the directional focus of the explosive force. The explosive elements can be stacked on top of each other to facilitate placement into a single borehole, but it would also be possible to have the explosive elements arranged horizontally with respect to each other, or located in separate boreholes spaced at specific distances from each other.
One specific embodiment of the invention is a seismic method that comprises the steps of generating seismic waves by detonating an explosive device in a subterranean formation, wherein the explosive device comprises explosive elements that detonate in a time delayed sequence, and detecting the seismic waves and/or reflections thereof with seismic detectors.
Another embodiment of the invention is a geophysical survey system, comprising a seismic energy source that includes an explosive device that can consist of multiple explosive elements that are capable of being exploded at different times. The system includes a detonation control device that is capable of detonating the various explosive elements at different times. The system also includes a plurality of seismic detectors that are adapted to detect seismic waves and/or reflections generated when the seismic energy source explodes. The seismic detectors transduce an electrical signal representative of the seismic waves and the reflections of seismic waves they detect. The system can also comprise a data acquisition and processing system that is in communication with the seismic detectors, for example through electrical data cables or by wireless data transmission. The data acquisition and processing system can sample the electrical signals generated by the seismic detectors and produce data representative thereof, for example by sampling and summing the data collected.
In one embodiment of the geophysical survey system, the explosive device can include a housing and can be located in a borehole. The various explosive elements can be separated by barrier elements that can include inert material. The explosive elements can be individually equipped with a detonation device that can be activated by a detonation control device capable of imposing controllable time delays between individual detonations. The geophysical survey system can comprise a plurality of detonation devices that are each connected to an explosive element, and the detonation control device can detonate the elements at different times providing a pre-selected time delay between the individual detonation devices. The explosive device can comprise, for example, a plurality of explosive elements that include shaped charges and are located on top of each other in a cylindrical arrangement.
Another embodiment of the invention is a downhole seismic source for generating an energy pulse comprising a detonator and a shaped charge device within a housing. The shaped charge device comprising an explosive element and a plurality of shaped charges arranged to focus the energy pulse in predetermined orientation. The shaped charges can be positioned in what is referred to as a cascading configuration. They can be arranged in a generally vertical orientation with each other and connected longitudinally to each other and can thereby define a common longitudinal bore within the shaped charge device. The shaped charges are capable of collapsing in a sequential order upon the detonating of the explosive element, thereby forming a jet or a metallic mass slug. This sequential collapsing of the shaped charges is capable of imparting an extended directed energy pulse that can be coupled to the earth thereby producing seismic waves.
Yet another embodiment is a seismic method comprising the steps of providing an explosive device comprising a housing enclosing a plurality of shaped charges. The explosive element is detonated thus collapsing the shaped charges and forming an extended directed force. This force is imparted onto a subterranean formation, creating seismic waves within the subterranean formation. The seismic waves and/or reflections are then detected with seismic detectors. The directed force can comprise a plurality of jets or slugs of metallic mass. In this method the shaped charges collapse in sequential order and there exists a time delay between the collapse of the individual shaped charges. The time delay between the collapse of the individual shaped charges is determined by the design and placement of the shaped charges in relation to the explosive element.
Still another embodiment of the invention is an apparatus for generating seismic impulses comprising an explosive element, a detonator and a mass flyer element located within a housing. The mass flyer may be attached to the explosive element and is capable of moving from an upper position to a lower position. When the explosive element is detonated it produces expanding gases. The apparatus can further comprise a spring that is affixed to the bottom of the housing and contacts the mass flyer when the mass flyer is in its lower position.
An embodiment of the invention is a method for generating seismic impulses comprising providing an apparatus comprising an explosive element and a mass flyer element located within a housing. The mass flyer is accelerated from an upper position towards a lower position where the mass flyer impacts onto the bottom of the housing creating a force that is imparted onto a subterranean formation. This force creates seismic waves and the seismic waves and/or reflections thereof are then detected with seismic detectors. A spring can be affixed to the housing whereby the spring contacts the mass flyer when the mass flyer is in its lower position where the spring decelerates the mass flyer upon contact with the moving mass flyer. The explosive element can be detonated, thereby producing expanding gases that propel the mass flyer towards its lower position.
Embodiments of the present invention have the ability to increase the duration of the explosion and thereby increase the quantity of low frequency seismic waves. This allows the use of less explosive material and/or shallower boreholes than in prior seismic methods.
An additional benefit of the current invention is that the explosive elements used in the seismic methods and the geophysical survey systems can be shaped to give greater directivity to the seismic energy.